This invention relates generally to communication in space and, more particularly, to communication systems using orbiting satellites to receive or transmit communication signals. Although there are a number of applications of the present invention, it will be best understood in the context of a space-borne communications relay linking a potentially large number of terrestrial terminals. Typically, such a relay includes an orbiting satellite with a large antenna system that can be physically or electronically directed toward selected terminals on the earth's surface. A communication from one terminal to another is directed up to the relay, which retransmits the communication toward the intended receiving terminal.
The antennas on communications relays are generally not large enough to direct transmissions to small regions of the earth's surface, or to communicate with transceivers of very low power. Typically, a relay antenna provides global coverage of practically the entire surface as viewed from the relay, and transceivers of moderate to high power must be used. Each transmission has to be allocated a separate frequency band to avoid interference between transmissions on overlapping frequency bands. The capacity of communication systems of this type would be greatly increased if larger antennas could be employed, because the theoretical gain of an antenna, and its ability to form a less, divergent beam, increases as the square of the antenna diameter. Unfortunately, the mass of a required antenna supporting structure also increases roughly as the square of the diameter. An antenna system of large mass burdens the launch vehicle needed to place the relay in orbit, and adds to the difficulty of controlling the relay once in orbit.
The performance of a large space-borne antenna is limited not only by the overall physical dimensions, but also by the ability to precisely control the locations of physical features of the antenna, such as reflector surfaces and feed elements. Larger antennas are more difficult to control and are more likely to have imperfections in their surface geometry. Any attempt to overcome these difficulties usually results in adding even more mass to the overall structure.
There is, therefore, a significant need for a communications relay that overcomes these problems and provides an antenna system having a large diameter and high gain, but without the usual penalty of increased mass. If a suitably large diameter and high antenna gain can be provided, the communication system can be designed around a frequency plan in which frequency bands are reused in transmissions that are angularly separated from each other. The resultant communication system would, therefore, be able to handle much larger volumes of message traffic within a given overall frequency allocation. In addition, a high antenna gain would permit the use of very small, low-power transceivers on the ground. The present invention achieves these and other objectives, as will become apparent from the following summary and more detailed description.